Birth of planet observed for first time

20 November 2015

Interview with

Professor David Rothery, Open University

So far, scientists have logged more than 1,900 planets orbiting other stars
planetoutside our solar system. But no one has yet spotted any that are still in the process of formation. Until now that is, because researchers in the US have finally spotted just such a planet. It's in orbit around a young 2-million-year-old star, which is 450 light years away from Earth and catchily called LkCa 15. This star is actually quite similar to our own Sun. Planetary scientist David Rothery, who wasn't involved in the study, explained the significance of the discovery to Connie Orbach...

David - What's exciting is we're seeing a planetary system in the act of a formation.  There are three planets orbiting this star; the nearest one's about ten times Earth's distance away from the star, so that's twice Jupiter's distance away from our sun, and the furthest one is about 20-25 astronomical units, so that's a little bit less than Neptune's distance from our Sun.  So three giant planets, and the innermost one has still got gas falling into it, which has been measured at this very high temperature of 10,000 degrees; its losing potential energy as it falls into the planet's gravity whirl and it gets really hot.  So we're seeing giant planets forming.

Connie - How does a planet form in the first place?

David - What happens is, as a star forms, it forms within a cloud of gas and dust which contracts gravitationally, and it contracts. To conserve angular momentum it must start spinning faster, so the gas and dust gets shaped into a disc rather than a three dimensional cloud, and within this disc you get concentrations of material which collapse onto each other to form planets.  Now the temperature is changing all the time this is going on, so you get the materials that can condense at high temperatures forming first; that's the metals and the rock, and later on you get ices forming, and then with a body that's iron and rock surrounded by ice, that's got enough gravity to start sucking the gas out of the cloud as well and probably we're seeing that stage occurring round LkCa 15.  You've got the kernel of a giant planet which is now scavenging all the gas that it can, and it's the in-fall of the gas which is shining so brightly and enabling the process to be imaged.

Connie - Why hasn't this been seen before?

David - Well, you need very sophisticated instrumentation to see this. The disc of gas and dust was seen round this star fifteen years ago. The innermost planet, LkCa 15b was imaged five years ago. Now they are using adaptive optics on the the large binocular telescope, and infrared filters to isolate different wavelengths of light that are being emitted as the gas falls inward and acretes onto the planet. So, it's a combination of big telescopes; very high resolution because you've got big lenses, and adaptive optics to sharpen the focus, and the right kind of spectral data to show the glow of the gas.

Connie - Oh wow!  So this is a really good example then of why we're wanting to get bigger and bigger telescopes, more and more technology. You know, they may cost a lot but they're showing us so much more.

David - Yes.  I've seen quotes suggesting that what we're seeing here is better than Hubble space telescope could ever achieve for example. It's not a mountain in Arizona, it's above a lot of the Earth's atmosphere which helps, it's got adaptive optics to increase the sharpness because the atmosphere is shimmering all the time, even from a cold mountain top in Arizona you need to correct for the atmospheric effects, and they are doing that very successfully now.

Connie - What can we learn from this sort of planetary formation?

David -   I think we're seeing an evolution in knowledge here.  This isn't going to revolutionise anybody's ideas, but we're able to demonstrate how early in the life of a star planets form, and maybe we can get some handle on the rate at which planets grow, because actually we don't really know that.  Some people suggest you can go from gas and dust to full size planets in a million years, some people say tens or hundreds of million years, and it would be good to gate a rate of processes.  That's what I'm looking forward to seeing from this but, of course, it's only one example, and it may not be typical of all planetary systems, but at least it's a start.

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